Fluorescent lamps are classified into a cold cathode fluorescent lamp (CCFL) having electrodes disposed inside of a cylindrical glass tube and an external electrode fluorescent lamp (EEFL) having electrodes disposed outside of the glass tube.
The fluorescent lamp includes a phosphor layer formed by coating a fluorescent material on an inner wall of the glass tube, and a discharge gas including a certain content of gas and mercury for driving the light emission in the glass tube. Particularly, since the EEFL has electrodes disposed on an outer wall of the glass tube instead of the inner wall, and it can be easily manufactured in the form of a capillary tube type.
Generally, when high voltage is applied to the electrodes, electrons in the glass tube collide with neutral gas atoms while traveling toward one of the electrodes (cathode electrode), thereby generating ions. The generated ions travel to the other electrode (anode electrode) so that secondary electrons are emitted from the anode electrode. By the discharge, in the case of a mercury fluorescent lamp, ultraviolet rays having a wavelength of about 253.7 nm are emitted by the collision of the traveling electrons with mercury atoms. The ultraviolet rays excite the phosphor layer to emit visible light.
However, when the fluorescent lamp is used for a long time, a darkening phenomenon occurs by an amalgam generated from the reaction of mercury filled in the fluorescent lamp with an alkali component of a glass substrate, or by an impurity of the discharge gas released from impure trace residues of the fluorescent substance.
Various methods have been proposed to suppress the darkening phenomenon.
Korean Pat. Publication No. 2001-0074017 discloses an EEFL, in which a metal oxide such as MgO, or CaO is deposited on an inner surface of the glass tube in order to increase the lifetime of the fluorescent lamp and the secondary electron emission. However, the patent 2001-0074017 only estimates the effect of using the metal oxide and does not prove the effect. Furthermore, the patent does not disclose a composition of the metal oxide. Moreover, a method for forming the ferroelectric layer is not provided.
Korean Pat. Publication No. 1999-0083535 discloses a fluorescent lamp wherein a passivation layer is interposed between the glass tube and the phosphor layer in order to suppress the darkening phenomenon and to keep a high luminous flux maintenance factor. That is, a colloid suspension is formed by dispersing γ-Al2O3 in water and then the colloid suspension is coated on a glass tube. Thereafter, the glass tube is baked at 600° C. so that the passivation layer is formed.
According to the publication No. 1999-0083535, only the case where the passivation layer is formed on a glass substrate is disclosed, rather than a capillary glass tube for the fluorescent lamp. Therefore, it is difficult to apply the technology disclosed in the patent to produce an EEFL having the cylindrical glass tube having a relatively small diameter. Also, it is difficult to have a uniform passivation layer due to deterioration of colloid stability, which is caused by aggregation of the γ-Al2O3 particles.
Therefore, a dry coating method such as a deposition or a sputtering may be used in order to form a uniform passivation layer. However, when the glass tube is a capillary type, it is impossible to form the passivation layer on the capillary type glass tube for the fluorescent lamp.